1. Field of the Invention
The present invention relates to a single-plate color projector and a method to adjust synchronization of a color wheel and an image forming device.
2. Description of the Related Art
FIG. 1 illustrates the basic configuration of a conventional single-plate color projector (also referred to as “color projector”) using a color wheel. In color projector 10 illustrated in FIG. 1, white light emitted from light source 11 is converged by reflective mirror 12, and irradiated to color wheel 13. Color wheel 13 comprises a disk which is formed with an optical filter (red filter) for transmitting only red light and reflecting or absorbing the remaining color light, an optical filter (green filter) for transmitting only green light and reflecting or absorbing the remaining color light, and an optical filter (blue filter) for transmitting only blue light and reflecting or absorbing the remaining color light. These filters are arranged along the circumferential direction of the disk. Color wheel 13 is driven by driving motor 23, controlled by motor control circuit 22, at high speeds. The filters in respective colors are sequentially inserted into a light path of the light emitted from light source 11 in association with the rotation of color wheel 13, so that white light irradiated to color wheel 13 is separated into red light, green light, and blue light in time series. Each of the separated red, green, and blue light is sent to light valve 14 then to reflective mirror 15 for irradiation to image forming device 16 such as a liquid crystal panel, DMD (registered trademark), etc. On the other hand, image forming device 16 is driven under the control of image forming device control circuit 17. Image forming device control circuit 17 drives image forming device 16 in accordance with a video signal applied thereto. Image forming device control circuit 17 turns ON or OFF each of the cells or miniature mirrors of image forming device 16 in accordance with a video signal applied thereto to reflect the irradiated single-color light pixel by pixel for optical modulation to form image light. The formed image light is directed into projection lens 18 which projects the image light onto a screen etc., not shown, in an enlarged view.
The color projector having the configuration described above must synchronize a color separation performed by color wheel 13 with an optical modulation performed by image forming device 16. In color projector 10 illustrated in FIG. 1, highly reflective marker 19 is attached onto color wheel 13, and reflection-type photo-interrupter 20 is disposed near color wheel 13. Reflection-type photo-interrupter 20 is a sensor having a light emitting element integrated with a light receiving element. Light emitted from the light emitting element impinges on and is reflected by reflective marker 19 each time color wheel 13 rotates once. The reflected light is then received by the light receiving element, and the light receiving element generates a signal. The signal generated by the light receiving element is detected by detector circuit 21, and is applied to motor control circuit 22 and image forming device control circuit 17 as a synchronizing signal. Motor control circuit 22 and image forming device control circuit 17 drive driving motor 23 and image forming device 16 respectively in accordance with the synchronizing signal applied thereto. The foregoing operation accomplishes the synchronization of the color separation performed by color wheel 13 with the optical modulation performed by image forming device 16. Details on a color projector which accomplishes the synchronization in the foregoing manner are described in the specification etc. of Japanese Patent Laid-open Publication No. 2001-337390 (reference document No. 1).
Another technique for accomplishing the synchronization of the color separation performed by the color wheel with the optical modulation performed by the image forming device may involve detecting a light color transmitted by the color wheel with a pair of sensors each detecting different color light, and detecting a time at which light beam from a light source passes through the boundary between the two color filters. For example, a pair of sensors may be composed of a photodiode for detecting blue light and a photodiode for detecting red light, and the beam spot is determined to have passed the boundary between the red filter and blue filter, when the outputs of the respective photodiodes become equal to each other, i.e., when a magenta color is detected. In this way, the boundary between the red filter and blue filter is detected as a reference position for the rotation of the color wheel, and the image forming device is driven based on the result of the detection, thereby accomplishing the synchronization of the color separation performed by the color wheel with the optical modulation performed by the image forming device. Details on a projector which accomplishes the synchronization in this manner are described in the specification etc. of Japanese Patent Laid-open Publication No. 127437/97 (reference document No. 2).
However, there will be errors in the position to which the reflective marker is attached, and the position at which the sensor is disposed. Further, the boundary between the respective color filters includes errors associated with manufacturing. When these errors exist, the technique described in reference document No. 1 fails to fully accomplish the synchronization of the color separation performed by the color wheel with the optical modulation performed by the image forming device, resulting in a failure in to correctly to reproduce the colors.
The technique described in reference document No. 2 cannot accurately detect a magenta color unless the pair of sensors (photodiodes) match each other in characteristics and are equal in detection sensitivity to each other, thus experiencing difficulties in detecting the reference position for the rotation with a high accuracy. Further, since the pair of sensors must detect color light under equal conditions, the pair of sensors are required to be disposed at optically equivalent positions. It is therefore difficult to detect the reference position for the color wheel with high accuracy in a simple configuration to control the rotation time. Consequently, colors are not correctly reproduced as is the technique described in the reference document No. 1.